1. Field of the Invention
The present invention relates to a sulfuric positive electrode for use in a lithium secondary battery and a method for manufacturing the same, and more particularly, to a sulfuric electrode having an enhanced electrode life cycle and a method for manufacturing the same.
2. Description of Prior Art
Portable electronic devices such as a notebook computer, a camcorder, a mobile phone and a compact recorder are being quickly developed. Accordingly, the portable electronic devices are widely being used and their demands are gradually increased. As a result, batteries that are energy sources became important. Among the batteries, a number of re-usable secondary batteries are sharply increased. In particular, a lithium secondary battery among the secondary batteries is most frequently being studied due to its high energy density and discharging voltage, and commercialized currently.
The most crucial portions in any battery as well as a lithium secondary battery are materials constituting negative and positive electrodes. In particular, a material that is used for the positive electrode of the lithium secondary battery should have a high discharging capacity, an inexpensive active material and an excellent electrode life cycle for a long-time use.
The positive electrode of a lithium sulfuric secondary battery has a theoretical capacity of 1,675 mAh/g, that is, a very high discharging capacity. The price of sulfur that is an active material is very low. Also, the lithium sulfuric secondary battery does not use heavy metal and thus is friendly environmental. However, the lithium sulfuric secondary battery has a very short electrode life cycle, which is a fatal demerit that is completely discharged within 30 cycles and is not charged and discharged any longer. The degeneration of the lithium sulfuric secondary battery has not been still clearly explained. Thus, there has not been clearly presented a method for enhancing an electrode life cycle. An experimental research for enhancing an electrode life cycle of the lithium sulfuric secondary battery has been performed by a processor, E. J. Cairns and his processor group in Berkley University, at Journal of Power Sources 89 (2000), pp21-226, in which solid electrolyte is changed from polyethylene-oxide (PEO) to polyethylene-glycol dimethyl ether. It can be seen that a discharging capacity is greatly decreased and the life cycle is little enhanced.
Under the circumstances, the inventors have discovered that an electrode life cycle can be enhanced without reducing a discharging capacity, when nickel (Ni) that is an electrical conductor is added to a sulfuric electrode material that is used as an active material of a lithium sulfuric battery that is promising as a next-generation lithium secondary battery positive material
To solve the prior art problems, it is an object of the present invention to provide a sulfuric positive electrode for use in a lithium secondary battery capable of enhancing an electrode life cycle without reducing a discharging capacity, by adding nickel that is an electrical conductor at the time of manufacturing a positive electrode of a lithium sulfuric secondary battery.
It is another object of the present invention to provide a method for manufacturing a sulfuric positive electrode for use in a lithium secondary battery capable of enhancing an electrode life cycle without reducing a discharging capacity, by adding nickel that is an electrical conductor at the time of manufacturing a positive electrode of a lithium sulfuric secondary battery.
To accomplish the above object of the present invention, according to the present invention, there is provided a method for manufacturing a sulfuric positive electrode for use in a lithium secondary battery, the method comprising the steps of: weighing sulfuric powder that is an active material and carbon powder and nickel powder that are electrical conductors and then ball-milling and mixing the weighed powder under an inactive atmosphere; stirring the mixed powder that is put in a solvent together with polyethylene-oxide (PEO), to thereby form a slurry for a positive electrode; and drying the slurry to obtain a sulfuric positive electrode.
According to another aspect of the present invention, there is provided a sulfuric positive electrode for use in a lithium secondary battery, which is manufactured by the above method.
The sulfuric positive electrode for use in a lithium secondary battery according to the present invention employs sulfur as an active material, polyethylene-oxide (PEO) as an ion conductor and a binder, and carbon and nickel as electrical conductors. Preferably, a small amount of lithium salt is additionally added.
The sulfur that is used as an active material for a sulfuric positive electrode in the present invention is directly reacted with lithium during discharging, to thereby form Li2S. Inversely, during charging, Li2S is phase-separated into Li and S, according to a reversible reaction. It is preferable that a content of sulfur is 10-90% with respect to the weight of the whole electrode composition. The reason is because 100% sulfur is an electrical non-conductor and a lithium ion has no conductivity, the electrode cannot function. Accordingly, the carbon that is an electrical conductor forms slurry together with a polymer such as polyethylene-oxide that is an ion conductor. Meanwhile, in the case that sulfur of less than 10% is included, that is, when the sulfur content is too excessively small, a fabricated battery cell has a reduced discharging capacity and thus is of no practical use.
In the present invention, it is appropriate that the materials added as an electrical conductor and an ion conductor are 5-45% with respect to the total weight of the electrode composition, respectively.
Nickel is added together with carbon as an electrical conductor. Carbon and nickel play roles of improving electrical conductivity of electrons within a positive electrode, that is, an anode. According to the research of the inventors, the added nickel forms NiS that is a nickel sulfide, and thus plays a role of a catalyst at the time of an electrode reaction of sulfur and lithium. As a result, it is expected that the life cycle of the electrode would prolong. It is appropriate that the content of the added nickel according to the present invention is within the scope of 10-30% with respect to the total weight of the electrode composition.
In the case that the nickel content is less than 10%, the content of the electrical conductor is too excessively small, and thus it is difficult to expect a sufficient electrical conduction effect. Meanwhile, in the case that the nickel content exceeds 30%, the content of the sulfur that is an active material is relatively decreased and thus the battery capacity is undesirably decreased.
Meanwhile, in the case of the nickel powder added as an electrical conductor in the present invention, as the size of the particle is more finite, the electrode life cycle becomes longer. Thus, it is preferable that the nickel powder has the size of 30 xcexcm or smaller in diameter. It is more preferable that the nickel powder has the size of 0.1-10 xcexcm.
PEO constituting a positive electrode is a solid polymer and plays a role of a binder as well as an ion conductor of a lithium ion.
Meanwhile, it is preferable that the positive electrode according to the present invention can further contain a small amount of lithium salt within the scope of 1-10%. The lithium salt is added in order to enhance the conductivity of the lithium ions in the anode. The representative lithium salt is LiCF3SO3.